The present invention relates to a finite conjugation large-aperture aspherical surface lens suitable for use in optical reading systems and optical communication transmission system.
Recently large-aperture aspherical surface lenses have been used in laser pickup systems of CD (Compact Disk) and LD (Laser Disk) devices. Furthermore, this type of lenses are finding spreading use in various fields such as opto-magnetic memories of computers, fine optical systems of optical communication transmission apparatus.
However, in most cases, infinite conjugation optical systems using a collimator lens have widely been used. For example, in U.S. Pat. No. 4,449,792 there was disclosed a large-aperture single lens with aspherical surfaces, and in U.S. Pat. No. 4,571,034 there was disclosed a lens system for optical recording type disks. In these inventions, however, there are following problems. The system requires a collimator lens and so the compactness and lightness thereof may not easily be obtained. In order to obtain a tiny spot size with large numerical aperture (NA), wave front aberration becomes large. On the other hand, amount of correction differs for both finite conjugation and infinite conjugation. Therefore, if an infinite conjugation lens is used in place of finite conjugation lens in correcting the spherical aberration, three-dimensional spherical aberration becomes extremely overcorrected and exceeds a diffraction limit. Thus they can not be used for practical use. Furthermore, such an optical system using infinite conjugation, however, is disadvantageous in that the production cost is high due to the use of a too large number of parts and due to a too long assembly time. Also, the lens weight is increased to hamper high-speed focusing operation. Thus, the above-mentioned optical system using the infinite conjugation was not practical.
Nowadays, plastic single lenses with aspherical surfaces are widely used in CD players or the like.
Plastic single lenses, however, are not recommended from the viewpoint of temperature characteristics. In addition, this type of lens is inferior in heat-resistance so that it cannot withstand high temperature developed during bonding of this type of lens to a lens mount through a bonding material such as powdered glass.
Precision-mold press technology has very recently been technically established in the manufacture of lenses, which has made it possible to produce aspherical surface lenses with a very high level of precision.
Infinite optical system requires two or more lenses when used in optical communication connector lens, in order to transmit a semiconductor laser beam to the end surface of optical fiber with a high coupling efficiency.
The present invention is based on a discovery of a possibility to realize a similar optical system by using finite system with a single lens formed by precision press-molding.